It is known to provide various cover assemblies adapted to extend over the open top of a rectangular bin or hopper. Such cover assemblies typically employ an elongated flexible cover having a length and width sufficient to completely cover the open top of the bin or hopper. A spring loaded roller is rotatably mounted at one side or end of the receptacle to which a first end of the cover is attached. A cable spool is rotatably mounted at the opposing end of the bin or hopper and is connected by elongated cables to the flexible cover. A drive motor causes the cable spool to rotate in such a way that the cover assembly has a first covered configuration in which the cover is unwound from the spring loaded roller and extends over the top of the bin or hopper, and a second uncovered configuration which the cover is wound about the spring loaded roller and the top of the bin or hopper is uncovered. These types of covers assemblies perform reasonably well in various situations e.g. when used to cover and uncover the open top of an elongated rectangular box such as an open bed trailer adapted to be moved by a truck along a highway. However, these forms of cover systems are not well suited for use with various types of equipment such as agricultural combine harvesters having open top hoppers leading into the harvester's grain tank. Bins or hoppers for these types of equipment are usually of a non-rectangular shape thus requiring specially shaped covers capable of extending over the open top of the harvester's grain tank. Specialty covers previously provided typically included a waterproof fabric intended to prevent rainwater and other debris from contacting grain that accumulates in the harvester's grain tank. Many such earlier covers were cumbersome structures requiring considerable time and the use of tools to assemble and disassemble same. Typical covers may include fabric that is poorly fitted thus allowing undesirable pools of water to collect in pockets defined between portions of a support frame. The accumulated water can eventually leak through and contact the grain tank. Still other prior art covers required considerable work to disassemble and remove same from the top of the harvester to provide clearance enabling the combine to pass beneath obstructions such as when passing through a barn door.